#include <iostream>
#include <cmath>
using namespace std;

int main()
{
	int n;
	cin >> n;
	double formula = 0.00;
	double f1 = pow((1 + pow(5,0.5))*0.5,n);
	double f2 = pow((1 - pow(5,0.5))*0.5,n);
	formula = 1.00 * (f1 - f2) / pow(5,0.5);
	
	printf("%.2lf",formula);
	return 0;
}
